Shape-memory polymers (SMPs) form a class of materials that can store and release elastic energy upon applying an external stimulus, such as heat or light. A shape-memory material can be deformed to a temporary shape and can return to its original shape after the application of the external stimulus. For example, a material heated above its shape-memory transition temperature, TSM, can be elastically deformed by subjecting it to external stresses and subsequently cooled, while under stress, beneath TSM. In the cooled state, external stresses can be removed and the material can retain its deformed shape. Upon heating above TSM, the material can recover its elastic strain energy and can return to its original shape. SMPs are noted for their ability to recover from large strains—up to several hundred percent—which can be imposed by mechanical loading. The large-strain recovery observed in SMPs is a manifestation of entropy elasticity.
SMPs can serve in biomedical devices such as vascular stents, clot-removal devices, catheters, programmable sutures, implants, and numerous other applications. Applications increasingly demand that shape-memory materials perform mechanical work against external loads; therefore, SMPs, in certain circumstances, should exhibit high shape energy densities. Other commercialization desires are diverse but can include: (i) a specified shape recovery stimulus (heat, light, chemical); (ii) ease of processability into different shapes; (iii) reproducible and robust shape-memory behavior upon cycling; and (iv) and low cost and straightforward scale-up.
One particularly desirable characteristic is a tunable shape recovery temperature, TSM, near the body's temperature. Most accessible thermally induced shape-memory polymers have a high modulus and require high triggering temperatures (Ttrig˜50-90° C.). For example poly(caprolactone) (PCL), poly(ω-pentadecalactone)(PPD), and poly(ester urethane) (PEU) have melting temperatures of ˜60° C., ˜75° C., and ˜45-60° C., respectively, and poly(lactide)(PLA) has a glass transition temperature (Tg) of ˜53° C. (Zotzmann J et al. Advanced Materials, 2010, 22(31), 3424-3429; Ahmad M et al. Macromolecular Chemistry and Physics, 2011, 212(6), 592-602; Xue L et al. Macromolecules, 2009, 42(4), 964-972). All these examples have thermal transitions well above the human body temperature (>35° C.), which prohibits these polymers from being useful for many potential biomedical applications.
There have been several attempts to realize body temperature triggered shape-memory behavior. For example, PEUs undergo phase-segregation into nanoscale hard and soft segment domains to form thermoplastic shape-memory materials that can resist mechanical creep. The shape-memory trigger temperature of these PEUs can be reduced by incorporating low molar mass crystallizable soft segments (Lendlein A and Langer R. Science, 2002, 296(5573), 1673-1676) or by manipulating the size of phase-segregated hard segment domains (Ahmad M et al. Macromolecular Chemistry and Physics, 2011, 212(6), 592-602; Xue L et al. Macromolecules, 2009, 42(4), 964-972). Another way to reduce the shape-memory transition temperature is to create a network polymer with well distributed net points. For example, Xu et al. prepared a network made of star-shaped PLA with a bulky and rigid nanoparticle POSS core, which reportedly lowered the excessive global entanglement of the tethered network chains and hence lowered the transition temperature by 20° C. (Xue J W and Song J. PNAS, 2010, 107(17), 7652-7657). While these approaches can have promise in some applications, new SMP's with transitions at or near body temperature are still needed. The methods and compositions disclosed herein address these and other needs.